The present invention relates to an optical modulator using a Mach Zehnder modulator and a light-modulation switching method.
Recently, in association with a rapid increase in amount of information, large capacity and long distance of an optical communication system is required. For example, an optical amplification repeating system of 40 Gb/s has been put into practical use. In the future, the larger capacity and the long distance will be further required. A TDM (Time Division Multiplexing) system and a WDM (Wavelength Division Multiplexing) system have been researched and developed.
With respect to an electro-optical modulator circuit in an optical communication system, an intensity modulation (direct modulation) is the most simple. In this modulation, light emission/quenching is controlled by directly turning on/off current flowing to a semiconductor laser in response to “0” and “1” of a data signal. However, the laser is directly turned on/off and the property of a semiconductor then causes chirping in an optical signal.
As a bit rate is higher, the chirping affects a harmful influence to transmission characteristics. Because an optical fiber has a property of wavelength division that signal light having different wavelengths has different propagation rates. The direct modulation causes the chirping, then the propagation rate is delayed and waveforms deteriorate during transmission through the optical fiber and the long-distance transmission and the fast transmission are not possible.
Therefore, in transmission at fast rates of 2.5 Gb/s and 10 Gb/s, such an external modulation is performed that a laser diode continuously emits light and an external modulator turns on/off continuous light generated from the laser diode on the basis of “0” and “1” of the data signal. As the external modulator, a Mach Zehnder (MZ) optical modulator is mainly used.
FIG. 20 is a block diagram showing the structure of a conventional optical modulator. Referring to FIG. 20, a conventional optical modulator 2000 comprises: a light source 2010; a MZ modulator 2020; and a MZ modulator 2030. The MZ modulator 2020 is a DQPSK (Differential Quadrature Phase Shift Keying) modulator that modulates carrier light output from the light source 2010 to a differential quadrature phase thereof.
The MZ modulator 2020 comprises two MZ modulators (I arm 2020A and Q arm 2020B), and performs the DQPSK by interference of the signal light phase-modulated by the MZ modulators with the phase difference of π/2. A bias supply unit 2021 supplies, to the MZ modulator 2020, a bias voltage corresponding to the DQPSK signal modulated by the MZ modulator 2020.
The MZ modulator 2030 is an RZ modulator that converts the signal light subjected to the DQPSK modulated by the MZ modulator 2020 into RZ (Return to Zero) pulses. A bias control unit 2031 supplies a bias voltage corresponding to the signal light subjected to the RZ-DQPSK modulated by the MZ modulator 2030 to MZ modulator 2030.
In addition to the RZ-DQPSK, external modulators using various-modulations are used in accordance with transmission conditions, e.g., an NRZ (Non Return to Zero) intensity modulation, CZRZ-DQPSK (Carrier Suppressed RZ-DQPSK) modulation, and Duobinary (Alternate mark inversion) modulation (refer to Japanese Laid-open Patent Publication No. 2000-162563 and Japanese Laid-open Patent Publication No. H3-251815).
For example, the RZ-DQPSK is advantageous to the long-distance transmission because of high proof strength of Polarization Mode Dispersion (PMD) and high Optical Signal Noise Ratio (OSNR) in oncoming transmission and reception. However, the spectrum of the signal light is wide.
Therefore, in the case of a small interval between wavelengths in a WDM transmission system comprising a repeater including a wavelength division multiplexing device, the signal is cut-off by the wavelength division multiplexing device, thereby increasing the penalty. Accordingly, the DQPSK or DPSK can be used in a short WDM-transmission path with a small interval between wavelengths and a transmission path through which a nonlinear optical effect is frequently caused.
However, with the above-mentioned conventional arts, the modulation is fixed depending on the type of modulator and the initial setting. Therefore, even if changing the transmission conditions of the optical communication system, such as the interval between the wavelengths in the WDM and the number of steps of the repeater, the modulation is not switched corresponding to the changed transmission condition. As a consequence, there is a program that transmission characteristics deteriorate depending on the transmission condition.
Further, if one optical communication system uses modulations varied depending on optical communication devices, the modulation needs to be matched to the optical communication device as the communication destination. However, the conventional arts cannot switch the modulation to that matching to the optical communication device as the communication destination. Therefore, there is a problem that the optical transmission is not possible between the optical communication devices using different modulations.
On the other hand, it is considered that a plurality of modulators corresponding to the modulations are arranged to switch a plurality of modulations. However, the arrangement of a plurality of modulators causes a problem of a large scale, a complicated structure, and an increase in costs of the device. Further, upon switching the modulation by manually switching the modulator, the switching of the modulator is troublesome. Therefore, there is a problem that it is not possible to flexibly cope with the optical communication system in which the transmission condition frequently changes.